Nutrient enrichment often alters the biomass and species composition of plant communities, but the extent to which these changes are reversible after the cessation of nutrient addition is not well‐understood. Our 22‐year experiment (15 years for nutrient addition and 7 years for recovery), conducted in an alpine meadow, showed that soil nitrogen concentration and pH recovered rapidly after cessation of nutrient addition. However, this was not accompanied by a full recovery of plant community composition. An incomplete recovery in plant diversity and a directional shift in species composition from grass dominance to forb dominance were observed 7 years after the nutrient addition ended. Strikingy, the historically dominant sedges with low germination rate and slow growth rate and nitrogen‐fixing legumes with low germination rate were unable to re‐establish after nutrient addition ceased. By contrast, rapid recovery of aboveground biomass was observed after nutrient cessation as the increase in forb biomass only partially compensated for the decline in grass biomass. These results indicate that anthropogenic nutrient input can have long‐lasting effects on the structure, but not the soil chemistry and plant biomass, of grassland communities, and that the recovery of soil chemical properties and plant biomass does not necessarily guarantee the restoration of plant community structure. These findings have important implications for the management and recovery of grassland communities, many of which are experiencing alterations in resource input.
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Abstract Aims The productivity–plant diversity relationship is a central subject in ecology under debate for decades. Anthropogenic disturbances have been demonstrated to affect productivity and plant diversity. However, the impact of disturbances on the productivity–diversity relationship is poorly understood.
Location An old‐field located at the Touch of Nature Environmental Center in Jackson County, Illinois, USA.
Methods A manipulative experiment with fertilizer (unfertilized, fertilized annually, fertilized every five years) and mowing (unmowed, mowed in spring only, mowed in spring and fall) in a successional old‐field began in 1996 to examine disturbance effects on above‐ground net primary productivity (ANPP)–plant diversity relationships. Taxonomic (species richness, T0) and phylogenetic (net relatedness index, NRI) diversity were selected as potential plant diversity metrics.
Results A unimodal relationship of ANPP with T0 and a negative relationship between ANPP and NRI were found across all treatments and years in this study, but individual years showed different patterns. Fertilization did not affect T0, NRI, and ANPP, whereas mowing stimulated T0 and ANPP but reduced NRI (i.e., increasing phylogenetic diversity) across all survey years. New colonists, especially exotic species introduced under mowing, but not locally extinct species, were more distantly related to resident species than by chance, implying that invasion of exotic species contributes to phylogenetic overdispersion of community assembly in the old‐field. However, the patterns of the unimodal relationship of ANPP with T0 and the negative correlation between ANPP and NRI did not change under fertilization or mowing in this study.
Conclusions Anthropogenic disturbances alter productivity and different dimensions of plant diversity, but do not change the patterns of the productivity–diversity relationships. Our findings highlight the robust relationship between productivity and diversity providing empirical support for productivity as a powerful predictor of plant diversity under intensified human activities.
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Abstract Anthropogenic environmental changes are known to affect the Earth's ecosystems. However, how these changes influence assembly trajectories of the impacted communities remains a largely open question.
In this study, we investigated the effect of elevated nitrogen (N) deposition and increased precipitation on plant taxonomic and phylogenetic β‐diversity in a 9‐year field experiment in the temperate semi‐arid steppe of Inner Mongolia, China.
We found that both N and water addition significantly increased taxonomic β‐diversity, whereas N, not water, addition significantly increased phylogenetic β‐diversity. After the differences in local species diversity were controlled using null models, the standard effect size of taxonomic β‐diversity still increased with both N and water addition, whereas water, not N, addition, significantly reduced the standard effect size of phylogenetic β‐diversity. The increased phylogenetic convergence observed in the water addition treatment was associated with colonizing species in each water addition plot being more closely related to species in other replicate plots of the same treatment. Species colonization in this treatment was found to be trait‐based, with leaf nitrogen concentration being the key functional trait.
Synthesis. Our analyses demonstrate that anthropogenic environmental changes may affect the assembly trajectories of plant communities at both taxonomic and phylogenetic scales. Our results also suggest that while stochastic processes may cause communities to diverge in species composition, deterministic process could still drive communities to converge in phylogenetic community structure.
